Dual valve well control in underbalanced wells

ABSTRACT

A method of isolating a reservoir of production fluid in a formation comprises providing a pair of valves ( 14, 16 ) in a bore intersecting a production formation and in which the hydrostatic pressuer in the bore at the formation is normally lower than the formation pressure, and then ontrolling the valves ( 14, 16 ) from surface such that the valves ( 14, 16 ) will only move from a closed configuration to an open configuration on experiencing a predetermined differential pressure across the valves.

This invention relates to well control, and in particular to a methodand apparatus for use in controlling access and flow to and from asubsurface well.

In the oil and gas exploration and production industry, bores aredrilled to access subsurface hydrocarbon-bearing formations. The oil orgas in the production formation is under pressure, and to preventuncontrolled flow of oil or gas from the formation to the surface, thatis a “blowout”, it has been conventional to fill the bore above theformation with fluid of sufficient density that the hydrostatic pressurehead provided by the column of fluid retains the oil or gas in theformation. However, it has been recognised that this practice may resultin damage to the formation, and may significantly reduce theproductivity of the formation. This problem has recently come to thefore as deeper and longer bores are drilled, and thus the hydrostaticpressure of drilling fluid or “mud” increases, and further as thepressures necessary to circulate drilling fluid and entrain cuttings inthe conventional manner increases.

One result of these experiences and findings has been the development oftechnology and methods which permit “under-balanced” drilling, that is adrilling operation in which the pressure of the drilling fluid is lowerthan the formation fluid pressure, such that oil and gas may flow fromthe formation and commingle with the drilling fluid. The fluids traveltogether to the surface and are separated at surface. In many cases, useof underbalanced drilling has resulted in marked increases in wellproductivity.

However, one difficulty associated with underbalanced drilling is therelatively high fluid pressures that are experienced at surface. Thisplaces an increased reliance on surface sealing arrangements, andgenerally increases the difficulty in controlling the well; theconventional high density fluid column is not present, and in the eventof difficulties, pumping higher density fluid into the well to “kill” orcontrol the well may take some time and is likely to result in damage tothe formation, perhaps to an extent where the well must be abandoned.

There is also a difficulty associated with making up drill string andthe like to be run into such wells, or indeed in any well where thepressure at surface is relatively high. In such wells, the relativelyhigh fluid pressure (which may be several hundred atmospheres) will tendto push the drill string up and out of the well, such that making upsuch a string becomes a difficult and potentially dangerous operation.This difficulty persists until the weight of the string is sufficient tocounteract the pressure force.

It has been proposed to avoid or overcome at least some of thesedifficulties by placing a flapper valve in a lower section of a well,the valve closing when the pressure forces acting from below the valveare greater than the pressure forces acting from above the valve. Thisplaces restrictions of the placement of the valve which, to beeffective, must be located close to the pressure balance point in thewell, that is the point where the upward acting fluid pressure force, orreservoir pressure, equals the downward acting force from the pressurehead produced by the column of fluid in the bore. Further, while such avalve may assist in preventing uncontrolled flow from a formation, thevalve will not serve to protect a formation from damage or contaminationin the event that the pressure above the valve rises; in such asituation elevated pressure above the valve will tend to open the valve.Similarly, testing the valve presents difficulties, as higher testpressures will tend to open the valve, and therefore no pressure greaterthan reservoir pressure may be safely utilised, as a higher pressurewould run the risk of damaging the formation.

It is among the objectives of embodiments of the present invention toobviate or mitigate these disadvantages.

According to one aspect of the present invention there is provided amethod of isolating a reservoir of production fluid in a formation, themethod comprising:

providing a valve in a bore intersecting a production formation and inwhich the hydrostatic pressure in the bore at the reservoir is normallylower than the formation pressure; and

controlling the valve from surface such that the valve will only movefrom a closed configuration to an open configuration on experiencing apredetermined differential pressure thereacross.

The invention also relates to an apparatus for use in isolating areservoir of production fluid in a formation, the apparatus comprising:

a valve adapted for location in a bore intersecting a productionformation and in which the hydrostatic pressure in the bore at thereservoir is normally lower than the formation pressure;

first valve control means for permitting control of the valve fromsurface; and

second valve control means for permitting control of movement of thevalve from a closed to an open configuration in response to apredetermined differential pressure across the valve.

Preferably, the valve is controlled such that it will only open whenthere is little or no pressure differential across the valve. Thus, asthe valve opens there is little if any flow of fluid through the valveas the pressure equalises; opening the valve in the presence of apressure differential may result in the rapid flow of fluid through thevalve as it opens, with an increased likelihood of erosion and damage tothe valve. In under-balanced and live well applications this allows thevalve to hold pressure from one or both sides, and minimises the risk offormation damage or contamination when the pressure above the valve ishigher than the pressure below the valve. Further, this feature may beutilised to minimise the risk of uncontrolled flow of fluid from theformation, in the event of pressure below the valve being higher thanthe pressure above the valve.

The valve may be positioned above, at or below the pressure balancepoint.

Preferably, the valve is controlled from surface by fluid pressure, thecontrol fluid supply of gas or liquid being isolated from the wellfluid, for example in control lines or in a parasitic annulus. The valvemay include a control fluid piston, application of control fluid theretotending to close the valve. Preferably, the valve is further alsoresponsive to well fluid pressure, and in particular to the differentialwell fluid pressure across the valve, such that the closed valve willremain closed or will open in response to a selected control pressure incombination with a selected differential pressure. The valve may includea piston in communication with fluid below the valve and a piston incommunication with fluid above the valve; application of pressure to theformer may tend to close the valve, while application of pressure to thelatter may tend to open the valve. In a preferred embodiment, a selectedfirst control pressure will close the valve. Such a first controlpressure in combination with a higher pressure below the valve will tendto maintain the valve closed. Further, increasing the control pressurewill maintain the valve closed in response to a higher pressure abovethe valve. This facility also allows the applied control pressure to bebrought to a particular value, the pressure differential across thevalve to be minimised and the control fluid pressure then varied toallow the valve to open.

Preferably, the valve is a ball valve. However, the valve may also be aflapper valve, or indeed any form of valve appropriate to theapplication.

Preferably, the valve comprises two valve closure members, which may betwo ball valves, two flapper valves, or even a combination of differentvalve types. The valves may have independent operating mechanisms. Thevalve closure members may close simultaneously, or in sequence, andpreferably the lowermost valve member closes first. This allows thevalves to be pressure-tested individually. Sequenced closing may beachieved by, for example, providing the valve members in combinationwith respective spring packs with different pre-loads.

Preferably, the valve is run into a cased bore on intermediate orparasitic casing, thus defining a parasitic annulus, between theexisting casing and the parasitic casing, via which control pressure maybe communicated to the valve. The parasitic casing is sealed to thebore-lining casing at or below the valve, typically using a packer orother sealing arrangement. The parasitic annulus may be used to carryfluids, for example to allow nitrogen injection in the well below thevalve. For example, additional casing may be hung off below the valve toextend the parasitic annulus, and a pump open\pump closed nitrogeninjection valve provided to selectively isolate the parasitic annulusfrom the well bore annulus. In other embodiments the parasitic annulusmay be utilised to carry gas or fluid lift gas or fluid to a point inthe well above the valve, or even between a pair of valves. One or moreone-way valves may be provided and which may be adapted to open at aparasitic pressure in excess of that required to close the valve orperform pressure tests above the valve. Such an arrangement may beutilised to circulate out a column of well kill fluid, prior to openingthe valve, or alternatively to inject a fluid slug prior to opening thevalves, or to inject methanol from the parasitic annulus to preventhydrate formation.

The valve may be configured to allow the valve to be locked open, forexample by locating a sleeve in the open valve.

The valve may be configured to permit pump-though, that is, onexperiencing a sufficiently high pressure from above, the valve may bemoved, for example partially rotated in the case of a ball valve, topermit fluid flow around the nominally closed valve.

According to another aspect of the present invention there is providedan apparatus for use in isolating a reservoir of production fluid in aformation, the apparatus comprising:

a valve adapted for location in a bore intersecting a productionformation and in which the hydrostatic pressure in the bore at thereservoir is normally lower than the formation pressure; and

first valve control means for permitting control of the valve fromsurface,

the valve including two valve closure members, both valve closuremembers being adapted to hold pressure both from above and from below.

Preferably, the valve closure members are ball valves. Alternatively,the valve closure members are flapper valves.

Preferably, the valve closure members are independently operable.

These and other aspects of the present invention will now be described,by way of example, with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic illustration of apparatus for use in isolating areservoir in accordance with a preferred embodiment of the presentinvention, shown located in a well;

FIG. 2 is an enlarged sectional view of valves of the apparatus of FIG.1; and

FIG. 3 is a further enlarged sectional view of one of the valves of theapparatus of FIG. 1.

Reference is first made to FIG. 1 of the drawings, which is a schematicillustration of apparatus 10 for use in isolating a reservoir inaccordance with a preferred embodiment of the present invention, theapparatus 10 being shown located in a well 12. The illustrated wellfeatures three main sections, that is a 17½ inch diameter hole sectionlined with 13⅜ inch diameter casing, a 12¼ inch hole section lined with9⅝ inch casing, and an 8½ inch hole section lined with 7 inch casing;those of skill in the art will of course recognise that these dimensionsare merely exemplary, and that the apparatus 10 may be utilised in awide variety of well configurations. The apparatus 10 is located withinthe larger diameter first well section and comprises upper and lowervalves 14, 16. As will be described, the valves 14, 16 are similar, withonly minor differences therebetween. The valves are mounted on tubing 18which extends from the surface, through a rotating blow-out preventer(BOP) 20, an annular preventer 22, and a standard BOP 24. Anintermediate tubular connector 26 joins the valves 14, 16, and a furthersection of tubing 28 extends from the lower valve 16, through the 9⅝inch casing, to engage and seal with the upper end of the 7 inch casing.Thus, an isolated annulus 30 is formed between the valves 14, 16 and thetubing 18, 28, and the surrounding casing; this will be referred to asthe parasitic annulus 30.

The apparatus 10 will be described with reference to an under-balanceddrilling operation, and in such an application a tubular drill stringwill extend from surface through the valves 14, 16 and the tubing 18,28.

Reference is now also made to FIG. 2 of the drawings, which is anenlarged sectional view of the valves 14, 16, shown separated. Referencewill also be made to FIG. 3 of the drawings which is an enlargedsectional view of the lower valve 16. As the only differences betweenthe valves 14, 16 is the pre-loading on the valve closing spring and thearrangement of porting for valve control fluid, only one of the valves16 will be described in detail, as exemplary of both. The valve 16 is aball valve and therefore includes a ball 34 located within a generallycylindrical valve body 36, and in this example the ends of the body 36feature male premium connections 38 for coupling to the tubing section18 and the connector 26.

The ball 34 is mounted in a ball cage 40 which is axially movable withinthe valve body 36 to open or close the valve. The valve 16 isillustrated in the closed position. Above the cage 40 is an upper piston42 which is responsive to fluid pressure within the tubing 18 above thevalve 14, communicated via porting 43. Further, a power spring 44 islocated between the piston 42 and a top plate 46 which is fixed relativeto the valve body 36. Accordingly, the spring 44, and fluid pressureabove the ball 34, will tend to move the valve ball 34 to the openposition.

Below the cage 40 is a lower piston 48 which, in combination with thevalve body 36, defines two piston areas, one 50 in fluid communicationwith the parasitic annulus 30, via porting 51, and the other 52 incommunication, via porting 53, with the tubing below the valves 14, 16,that is the reservoir pressure

In use, in the absence of any pressure applied to the valves 14, 16 viathe parasitic annulus 30, the springs 44 will urge the valve balls 34 tothe open position, allowing flow through the valves 14, 16. If howeverit is desired to close the valve, the pressure in the parasitic annulus30 is increased, to increase the force applied to the parasitic pistons50. The pre-load on the spring 44 in the lower valve 16 is selected tobe lower than the pre-load of the spring 44 in the upper valve 14, suchthat the lower valve 16 will close first. Thus, the effectiveness of theseal provided by the lower valve 16 may be verified. A further increasein pressure in the parasitic annulus 30 will then also close the uppervalve 14.

The valve balls 34 are designed to permit cutting or shearing oflightweight supports such as slickline, wireline or coiled tubing,passing through the apparatus 10, such that the valves may be closedquickly in an emergency situation without having to withdraw a supportform the bore.

With the valves 14, 16 closed, the reservoir is now isolated from theupper section of the well. This facilitates various operations,including the retrieval, making up and running in of tools, devices andtheir support strings above the apparatus 10, or the circulation offluids within the upper end of the tubing 18 to, for example, fill thetubing 18 with higher or lower density fluid.

In the event that the reservoir pressure below the valves 14, 16 ishigher than the pressure in the tubing 18 above the valves 16, 18, thereservoir pressure acting on the pistons 52 will tend to maintain thevalves 14, 16 closed, thus preventing uncontrolled flow of formationfluids from the reservoir.

In the event that the pressure differential is reversed, that is thepressure force above the valves 14, 16 is greater than the reservoirpressure acting below the valves 14, 16, the parasitic pressure may beincreased to increase the valve closing force acting on the pistons 50,to counteract the valve opening force acting on the pistons 42.

The area of the upper piston 42 is equal to the combined areas of theparasitic and reservoir pistons 50, 52, while the parasitic piston 50 islarger than the reservoir piston 52. Thus, if it is desired to open thevalve from a closed position, this is normally achieved by increasingthe pressure in the parasitic annulus 30 to a point where the parasiticpressure is substantially similar to the reservoir pressure. Thepressure in the tubing 18 is then increased, and as the tubing pressureapproaches the reservoir pressure the forces acting on the pistons 42reach a level similar to the oppositely acting forces on the lowerpistons 48, such that the springs 44 will tend to open the valves whenthe parasitic pressure is vented at surface.

While the parasitic pressure remains vented, the springs 44 will retainthe valves open.

With this arrangement it would be possible to open the valves when thetubing pressure above the valves 14, 16 was lower than reservoirpressure, if the parasitic pressure was not increased to be greater orequal to the reservoir pressure. However, this would result in thevalves 14, 16 opening with a pressure differential, and the resultingrapid flow of fluid through the valves would bring an increaselikelihood of erosion and damage to the valves and upstream equipment.

In the event that one or both of the valves cannot be opened, and it isdesired to, for example, “kill” the well, it sufficient tubing pressureis applied from surface the valve balls 34 will be pushed downwardly toan extent that kill fluid may pass around the balls 34 and then out ofpump-through ports 54 provided in the lower ball seats 56.

If desired, one or more one-way valves may be provided in the tubing 28or valve body 36. For example, one or more one-way pressure reliefvalves may be provided above the upper valve 14, and configured to passgas or fluid from the parasitic annulus into the tubing 18. Such a valvepositioned just above or between the valves 14, 16 may be used to, forexample, circulate out a column of well kill fluid prior to opening thevalve, or to inject a fluid slug prior to opening the valves. Such avalve could also be used to inject methanol from the parasitic annulus30 on top of the upper valve 14 to prevent hydrate formation.Alternatively, a one-way valve could be incorporated between the valves14, 16. Of course, such a valve or valves would only open in response toa parasitic annulus pressure in excess of that required to close thevalves, to perform a pressure test from above a closed valve, or tosupport a column of well kill fluid above the valves.

In the illustrated embodiment the provision of the parasitic annulus mayalso be used to advantage to, for example, allow nitrogen injection inthe well below the apparatus 10. For example, a nitrogen injection pointcould be provided on the tubing 28 below the apparatus 10. Of course theinjection point would have to be isolated from the tubing bore using apump open\pump close nitrogen injection valve.

From the above description it will be apparent to those of skill in theart that the apparatus described above provides a safe and convenientmethod of isolating a reservoir, and the ability of the valves to holdpressure from both above and below is of considerable advantage to theoperator, and provides additional safeguards and convenience inunder-balanced drilling, at balance drilling or live well\light weightintervention environments, most particularly in the deployment ofdrilling assemblies, intervention assemblies, workover assemblies,completions, liners, slotted liners or sandscreens.

Those of skill in the art will also recognise that the illustratedembodiment is merely exemplary of the present invention, and thatvarious modifications and improvements may be made thereto withoutdeparting from the scope of invention. For example, rather thancontrolling the operation of the valves 14, 16 via the parasitic annulus30, conventional control lines may be run from surface to supply controlfluid to the valves. Further, rather than providing valves in individualhousings, a common housing assembly for both valves could be provided.The above described valve arrangements rely primarily on metal-to-metalseals between the balls and the valve seats, and of course in otherembodiments elastomeric seals may also be provided. The valvesillustrated and described above are in the form of ball valves, thoughthose of skill in the art will recognise that flapper valves may also beutilised, particularly flapper valves having the facility to be heldclosed in response to both pressure from above and from below.

1. A method of isolating a reservoir of production fluid in a formation,the method comprising: providing a valve in a bore intersecting aproduction formation and in which the hydrostatic pressure in the boreat the formation is normally lower than the formation pressure, whereinthe valve is initially open; positioning the valve below the pressurebalance point; applying a selected first control pressure to close thevalve, wherein the first control pressure in combination with a higherpressure below the valve maintains the valve closed; and controlling thevalve from surface such that the valve will move from a closedconfiguration to an open configuration only at a predetermineddifferential pressure thereacross.
 2. The method of claim 1, wherein thevalve is controlled such that it will only open when there is little orno pressure differential across the valve.
 3. The method of claim 2,wherein the bore is formed by underbalanced drilling.
 4. The method ofclaim 1, wherein the closed valve is controlled to hold higher pressureabove the valve.
 5. The method of claim 1, wherein the closed valve iscontrolled to hold higher pressure below the valve.
 6. The method ofclaim 1, wherein the closed valve is controlled to hold pressure fromboth sides.
 7. The method of claim 1, wherein the valve is controlledfrom surface by fluid pressure.
 8. The method of claim 1, wherein acontrol fluid supply is supplied from surface to the valve through atleast one control line.
 9. The method of claim 1, wherein a controlfluid supply is supplied from surface to the valve through a parasiticannulus.
 10. The method of claim 1, further comprising applying a higherpressure below the valve to maintain the valve closed, without continuedapplication of said control pressure.
 11. The method of claim 1,comprising increasing said control pressure to maintain the valve closedin response to a higher pressure above the valve.
 12. The method ofclaim 1, comprising bringing the applied control pressure to aparticular value, minimizing the pressure differential across the valve,and then varying the control fluid pressure to open the valve.
 13. Themethod of claim 1, further comprising locking the valve open.
 14. Themethod of claim 1, comprising providing two similar valves in the bore.15. The method of claim 14, further comprising closing the valvessimultaneously.
 16. The method of claim 14, further comprising closingthe valves in sequence.
 17. The method of claim 16, further comprisingclosing a lowermost valve first.
 18. The method of claim 16, furthercomprising initially closing a lowermost valve.
 19. The method of claim1, comprising running the valve into a cased bore on intermediate orparasitic casing, thus defining a parasitic annulus between the existingcasing and the parasitic casing.
 20. The method of claim 19, furthercomprising sealing the parasitic casing to the bore-lining casing at orbelow the valve.
 21. The method of claim 20, further comprising carryingfluids into the bore below the valve through the parasitic annulus. 22.The method of claim 21, wherein the fluid is nitrogen and the nitrogenis injected in the bore below the valve.
 23. The method of claim 20,further comprising carrying gas, fluid lift gas or fluid to a point inthe bore above the valve.
 24. The method of claim 20, further comprisingproviding at least one one-way valve between the parasitic annulus andthe bore and opening the one-way valve in response to a parasiticpressure in excess of that required to function the valve or performpressure tests on the valve.
 25. The method of claim 24, furthercomprising circulating out a column of well kill fluid above the valvevia the parasitic annulus and the one-way valve prior to opening thevalve.
 26. The method of claim 24, further comprising injecting a fluidslug via, the parasitic annulus and the one-way valve prior to openingthe valve.
 27. A method for controlling a pressure surge in a string ofdown hole tubulars comprising; closing a first valve in response to thepressure surge; opening the first valve by application of a first fluidpressure from the surface; closing a second valve in response to theapplication of the first fluid pressure; and opening the second valve inresponse to a second fluid pressure applied from the surface.
 28. Anapparatus for use in isolating a reservoir of production fluid in aformation, the apparatus comprising: a valve system in a productiontubular having: a first valve having: a first valve control forpermitting control of the first valve from surface; and a second valvecontrol for permitting control of movement of the first valve from aclosed to an open configuration in response to the predetermineddifferential pressure across the first valve; and a second valve,wherein each valve is controlled with a differential pressure across thevalve and the differential pressure that controls the first valve is thefluid pressure outside the production tubular and the fluid pressureinside the production tubular.
 29. The apparatus of claim 28, whereinthe first valve control is operable to move the first valve from theopen configuration to the closed configuration.
 30. The apparatus ofclaim 28, wherein the first valve is adapted to hold pressure from atleast one side.
 31. The apparatus of claim 28, wherein the first valveis adapted to hold pressure from both sides.
 32. The apparatus of claim28, wherein the first valve control is responsive to the fluid pressureoutside the production tubular.
 33. The apparatus of claim 28, furthercomprising a parasitic casing for defining a control fluid-carryingparasitic annulus.
 34. The apparatus of claim 28, wherein the first andsecond valves have independent operating mechanisms.
 35. The apparatusof claim 28, wherein in the open configuration the first valve allowsfor passage of downhole tools.
 36. A method of isolating a reservoir ofproduction fluid in a formation, the method comprising: providing avalve in a bore intersecting a production formation and in which thehydrostatic pressure in the bore at the formation is normally lower thanthe formation pressure, wherein the valve is initially open; applying aselected first control pressure to close the valve, increasing the firstcontrol pressure to maintain the valve closed in response to a higherpressure above the valve; and controlling the valve from surface suchthat the valve will move from a closed configuration to an openconfiguration only at a predetermined differential pressure thereacross.37. A method of isolating a reservoir of production fluid in aformation, the method comprising: providing a valve in a boreintersecting a production formation and in which the hydrostaticpressure in the bore at the formation is normally lower than theformation pressure, wherein the valve is initially open; applying aselected first control pressure to close the valve, controlling thevalve from surface such that the valve will move from a closedconfiguration to an open configuration only at a predetermineddifferential pressure thereacross; bringing the first control pressureto a particular value, minimizing the pressure differential across thevalve; varying the control fluid pressure to open the valve.
 38. Amethod of isolating a reservoir of production fluid in a formation, themethod comprising: providing a valve in a cased bore intersecting aproduction formation and in which the hydrostatic pressure in the boreat the formation is normally lower than the formation pressure; runningthe valve into the cased bore on an intermediate casing, wherein anannulus is defined between the existing casing and the intermediatecasing; sealing the intermediate casing to the existing casing at orbelow the valve; carrying fluids into the cased bore below the valvethrough the annulus, wherein the fluid is nitrogen and the nitrogen isinjected in the cased bore below the valve; controlling the valve fromsurface such that the valve will move from a closed configuration to anopen configuration only at a predetermined differential pressurethereacross.
 39. A method of isolating a reservoir of production fluidin a formation, comprising; providing a valve in a bore intersecting aproduction formation and in which the hydrostatic pressure in the boreat the formation is normally lower than the formation pressure; runningthe valve into a cased bore on intermediate or parasitic casing, thusdefining a parasitic annulus between the existing casing and theparasitic casing; sealing the parasitic casing to the bore-lining casingat or below the valve; carrying fluids into the bore below the valvethrough the parasitic annulus, wherein the fluid is nitrogen and thenitrogen is injected in the bore below the valve; and controlling thevalve from surface such that the valve will move from a closedconfiguration to an open configuration only at a predetermineddifferential pressure thereacross.
 40. A method of isolating a reservoirof production fluid in a formation, the method comprising: providing avalve in a bore intersecting a production formation and in which thehydrostatic pressure in the bore at the formation is normally lower thanthe formation pressure, wherein the valve is initially open; positioningthe valve at the pressure balance point; applying a selected firstcontrol pressure to close the valve, wherein the first control pressurein combination with a higher pressure below the valve maintains thevalve closed; and controlling the valve from surface such that the valvewill move from a closed configuration to an open configuration only at apredetermined differential pressure thereacross.
 41. A method ofisolating a reservoir of production fluid in a formation, the methodcomprising: providing a first valve in a bore intersecting a productionformation and in which the hydrostatic pressure in the bore at theformation is normally lower than the formation pressure, wherein thefirst valve is initially open; providing a second valve in the bore;applying a selected first control pressure to close the first valve,wherein the first control pressure in combination with a higher pressurebelow the first valve maintains the first valve closed; and closing thesecond valve after the first valve; controlling the first valve fromsurface such that the first valve will move from a closed configurationto an open configuration only at a predetermined differential pressurethereacross.